Current state-of-the-art detection and surgical resection tools used in cancer treatment are insufficient. Early stage disease can be missed, resection can be incomplete and these two factors alone are major contributors to morbidity and mortality. Outcomes are intrinsically linked to disease detection and treatment efficacy. Therefore, improvement in the detection of early cellular changes, as well as enhanced visualization of diseased tissue, is of paramount importance.
Optical methods continue to provide a powerful means for studying cell and tissue function. Recent discoveries in Molecular Imaging (MI) are certain to play a vital role in the early detection, diagnosis, and treatment of disease. MI will also aid in the study of biological and biochemical mechanisms, immunology, and neuroscience. MI agents commonly consist of a signaling moiety (fluorophore, radioisotope or Gd3+ ion) and a targeting functionality such as an antibody or peptide, sugar or a peripheral benzodiazepine receptor (PBR) ligand. NIR molecular imaging agents are particularly attractive due to the inherently low water and tissue absorption in the NIR spectral region. Additionally, the low scattering cross-section and lack of autofluorescence background in the near infrared (NIR) region facilitate deep penetration and high-resolution images from small interrogated volumes.
While glandular and secretory tissues are normally rich in PBR, other quiescent tissue ordinarily express PBR at relatively low levels. Primarily spanning the bi-layered mitochondrial membrane, the PBR is expressed almost ubiquitously and thought to be associated with many biological functions including the regulation of cellular proliferation, immunomodulation, porphyrin transport, heme biosynthesis, anion transport, regulation of steroidogenesis and apoptosis. Given the importance of PBR toward regulating mitochondrial function, it is not surprising that PBR density changes have been observed in acute and chronic neurodegenerative states in humans, as well as numerous forms of cancer. For example, temporal cortex obtained from Alzheimer's patients showed an increase in PBR, and correlations with Huntington's disease, multiple sclerosis and gliosis have been demonstrated. Breast cancer generally demonstrates increased PBR expression and represents another potentially attractive target, especially in the NIR. The development of high affinity ligands for PBR (such as, for example, PK-11195, Ro5-4864, DAA1106, and DAA1107) has made non-invasive imaging modalities more suitable.
Other functional imaging targets include the glucose transporter and thymidine kinase 1. By targeting the glucose transporter, [18F]-fluoro-deoxyglucose (FDG) has been successfully employed as a positron emission tomography (PET) agent to determine the metabolic statues (cellular respiration) of suspect tissues. Modest functionalization of glucose at the C-2 position does not hinder sugar uptake but does prevent cellular metabolism, therefore glucose agents can accumulate intracellularly. Since tumor cells metabolize glucose a higher rate than normal cells, the accumulation of glucose mimics (i.e. FDG and similar agents) can facilitate discrimination of tissues based on their metabolic status. While FDG imaging certainly has demonstrated utility to the clinical oncologist, the requirement of a cyclotron and a PET scanner somewhat limit its use.
Recently, in effort to improve the specificity of functional imaging agents like FDG, new probes for cellular proliferation imaging have been developed. Targeting the enzyme thymidine kinase 1 (TK1), an enzyme responsible for DNA replication, [18F]3′-deoxy-3′fluorothymidine (FLT) has been shown to be an attractive complement to FDG imaging. Similar to FDG, FLT is not fully metabolized by cells and accumulates in target tissues, making it a promising imaging agent for rapidly proliferating tissues. When used in combination with FDG, clinical imaging of diseased tissue has the ability to be highly sensitive and specific.
It has been shown that NIR emitting Ln-Chelates can be prepared opening the avenue to complexes with spectral properties more compatible with biological imaging such as visible absorption, NIR emission and microsecond-long emission lifetimes. These complexes have high molar absorptivity and have luminescent lifetimes in the microsecond regime allowing temporal rejection of noise.
The present inventors have demonstrated the synthesis and utility of Eu-PK11195 and Gd-PK11195. Others have prepared PK11195 as a PET agent for use in humans. A NIR Pyropheophorbide agent has been reported for imaging glucose transporters, however this agent was not spectroscopically optimized for deep tissue in-vivo imaging (ex. 679 nm, em. 720 nm). At present, the authors are unaware of any NIR imaging agents based on thymidine imaging.